Green plants and photosynthetic bacteria capture and utilize sunlight by means of molecular electronic complexes, reaction centers, that are embedded in their membranes. In oxygenic plants, photon capture and conversion of light energy into chemical energy take place in pigment-protein complexes known as Photosystem I ("PSI") and Photosystem II ("PSII") reaction centers. As explained in Nature, R. Hill and F. Bendall (London) 186, 136 (1960), photosynthesis requires PSII and PSI working in sequence, using water as the source of electrons and CO.sub.2 as the terminal electron acceptor. The two reaction centers use a special pair of chlorophyll molecules as the primary electron donor and chlorophyll or pheophytin as the primary electron acceptor. Following excitation, transfer of an electron from the excited primary donor to the primary acceptor occurs within picoseconds, a process characterized by high quantum efficiency and minimal side reactions. For the PSI reaction center, the midpoint oxidization potential generated by the primary electron donor (P700) is about +0.4 V and the corresponding reduction potential generated by the electron acceptor (4Fe-4S center) is about -0.7 V. The PSI reaction center, therefore, is a photodiode (unidirectional electron flow) and nanometer-sized (.about.6 nm) solar battery.
Additionally, since the PSI reaction center is one of the pigment-protein complexes that is responsible for the photosynthetic conversion of light energy to chemical energy, these reaction centers may be used as an electronic component in a variety of different devices. These possible devices include, but are not limited to, spatial imaging devices, solar batteries, optical computing and logic gates, optoelectronic switches, photonic A/D converters, and thin film "flexible" photovoltaic structures. However, in order to make these PSI reaction centers into a molecular device it is very important to use a method to selectively immobilize the PSI reaction centers onto a substrate without denaturation of the PSI.
The prior art has attempted to replicate this photochemical reaction by using PSI reaction centers on a metal surface. The prior art has shown that platinum may be precipitated on the surface of photosynthetic membranes, thereby making direct electrical contact with the acceptor side of PSI, where it can either catalyze hydrogen evolution or drive photocurrent through an external circuit. Additionally, the prior art has recently shown that isolated PSI reaction centers can be platinized without altering their intrinsic excitation dynamics and initial electron transfer reactions. Additionally, the prior art has shown that a two-dimensional spatial array of isolated PSIs may be anchored onto a metal surface by PSI-platinum-gold bonding at a biological temperature and pH. However, these prior art methods were not capable of orienting the PSI reaction centers on the surface of the substrate. PSI reaction centers which are not oriented will cancel each other out, which, therefore, would not permit the PSI-immobilized devices to be used in photoelectric devices such as solar batteries or logic gates.
The present invention provides a solution to the orientation problem. The present invention provides a method for controlling the orientation of two-dimensional spatial arrays of isolated reaction centers on derivatized surfaces. By controlling the orientation of the reaction centers, these arrays may be used in the construction of biomolecular electronic devices.